1. Field of the Invention
The present invention relates to an optical disk apparatus which can record and reproduce high-density information for digital high definition video data or High-Vision video data for example. More particularly, the present invention relates to an optical disk apparatus which can realize an improved tilt control accommodating a state of inclination of the optical disk, and a method of tilt control.
2. Description of the Related Art
A disk drive for an optical disk is provided with an optical head (i.e., an objective lens) which is disposed so as to face a recording surface of the optical disk when the optical disk is inserted into the disk drive. In recent years, increasing recording density of the optical disk, in which recording and reproduction are realized with a short wavelength laser, requires that the inclination angle of the optical head be adequately compensated over the entire surface of the disk, i.e., entirely from an inner periphery to an outer periphery. One manner of compensation for the angle of inclination is called tilt control. A combination of the tilt control, a focus control and a tracking control (alignment/positioning control) of the optical head allows for recording and reproduction of information to and from a high-density optical disk.
One conventional technique of the tilt control implements the tilt control prior to the tracking as disclosed in FIG. 11 of Japanese Patent Application Laid-Open No. 2003-217153. For example, Japanese Patent Application Laid-Open No. 2003-217153 discloses the tilt control employing a push-pull tracking error signal for tilt compensation.
For the implementation of the tilt control as described above, a relation between an amount of correction of inclination and amplitude of a detected reproduction signal needs to be known corresponding to the inclination angle (tilt) of the optical head. On the other hand, Japanese Patent Laid-Open No. 2000-311368 discloses a so-called hill-climbing method according to which the tilt of the optical head is changed to both negative and positive angles and an angle where the correction amount is optimal is selected.
According to a third conventional technique disclosed in Japanese Patent Application Laid-Open No. 2001-307359, a tilt error signal is generated based on a difference between a differential phase detection (DPD) signal and a push-pull signal for the tilt control. Here, the DPD signal is obtained through: finding each sum of signals supplied from diagonally arranged photodetectors, comparing phases of two acquired signals, and using the phase difference as a track position error signal. When the tilt error signal is generated based on the difference between the DPD signal and the push-pull signal, both the DPD signal and the push-pull signal are offset due to the tilt, and influenced also by a shift of the objective lens. The third conventional technique regards the influence of the tilt on the DPD signal as insignificant, and finds the difference between the DPD signal and the push-pull signal in order to offset the influence of the lens shift, thereby allowing detection of only a component of the tilt that has a different influence on the DPD signal and the push-pull signal.
When the first conventional technique is applied, for example, to an optical disk which is 12 centimeters (cm) in diameter and capable of storing the Hi-Vision data using a blue laser, however, the amount of tilt is difficult to be distinguished from the lens shift factors. Hence the detection of the amount of tilt from the push-pull signal is not a realistic solution. Further, the second conventional technique, dissimilar to the first conventional technique, performs the tracking control before the tilt control and utilizes the amplitude of the reproduction signal obtained in the tracking control in the tilt control (see FIG. 4 of the cited document). In such case, the accuracy of the tilt control might be deteriorated by the influence of the tracking control. Still further, the third conventional technique needs to simultaneously detect the DPD signal and the push-pull signal, and a circuit structure for realization of such detection is significantly complicated.
In addition, in the optical disk intended for high-density information recording such as the 12 cm optical disk capable of storing High-Vision video data using a blue laser, the amount of tilt is difficult to detect and a favorable tilt control is not easy to realize. Such inconvenience arises not only in a disk with a single recording layer on one surface, but also in a disk with plural recording layers on one surface, in which laser reflectance decreases to complicate the problem.